Before I had my second child, I often had a menu plan for the week, but was inconsistent on the grocery shopping, since I could just wander down to the grocery store as a nice outing. That was then. Now, with a newborn and a two year old, a trip to the store is more complicated, and I have to be religious about meal plans. Here's what I do:
• check the fridge to see what needs using up (I try to keep a list of these things in an ongoing journal)
• add to my weekly meal plan any event that will affect meal timing (do we have a play group at 6PM? is my husband out for the evening?)
• sort through my favourite cookbooks to find interesting recipes (my current favourites are Mark Bittman's Food Matters and Canadian Living's Eat Well)
• outline meals, including side dishes and dessert
• make a grocery list based on these menus, and buy it all at once.
Important tip: don't overplan meals. While I need to have leftovers to use for lunches, it's wasteful to have more leftovers than you can use. I also don't particularly like leftovers after the first day of them, so I make an effort not to plan TOO much. Takes practice...